It should be remembered that in multicarrier modulation communications systems, in particular in discrete multitone (DMT) multicarrier modulation communications systems, the bandwidth used is divided into subbands SBi, with each subband modulating an independent carrier. The number of bits and the sending power assigned to a subband depend on the constraints imposed on the transmission line in terms of attenuation, stationary noise, etc., and thus vary from one subband to another.
Furthermore, it is known that a multichannel link can transport a certain number of streams FLj each associated with one channel of the link. For example, an ADSL link divided into seven different channels, namely four high bit rate unidirectional channels AS0, AS1, AS2, AS3 and three low bit rate bidirectional channels LS0, LS1, LS2, is adapted to transmit seven streams. The transport channels of the link must therefore be chosen as a function of the service corresponding to the stream concerned that is to be transported. Thus the downlink streams of Internet or digital television services are transmitted on AS unidirectional channels and Voice over IP (VoIP) or teleconference services and uplink streams of Internet or digital television services are transmitted on LS bidirectional channels.
It is therefore clear that, in multichannel link transmission systems such as ADSL transmission systems, a plurality of services can be offered simultaneously provided that the sum of the numbers Mj of bits to be transmitted in the streams FLj does not exceed the total bit rate that has been configured for the link.
After synchronization and multiplexing and before distribution of the bits to the subbands SBi, the streams are organized into frames so that the bits of each stream to be transmitted are grouped together and succeed each other in the frame. The following order might therefore be encountered in a frame: M1 bits of the stream FL1 of an Internet service on channel AS0, then M2 bits of the stream FL2 of a digital television service on channel AS1, then M3 bits of the stream FL3 of a VoIP service on channel LS0, then M4 bits of the stream FL4 of a video conference service on channel LS1, and so on.
The distribution of the bits Mj of the streams FLj between the subbands SBi is conventionally effected in two stages.
In a first stage, by means of rate adaptive (RA) binary allocation algorithms known in themselves, each subband SBi is assigned a number ni of bits that can be transmitted in that subband, given the signal-to-noise ratio of the transmission line for the subband and given also a noise margin Γ. The higher this noise margin, the more robust the transmission to the various sources of interference on the line, in particular impulse interference, and the higher the quality of service QoS.
A second stage of the distribution procedure allocates the successive bits of the streams of the frame to the subbands SBi in increasing order of their number ni of bits. This method of allocating bits is known as the tone ordering method.
A consequence of the nature of the binary allocation algorithms currently used is that the various streams of an ADSL link, for example, can be transmitted only with exactly the same noise margin Γ, i.e. with the same quality of service QoS.
Now, with some services, such as the VoIP or videoconference services referred to above by way of example, there is an a priori necessity for a higher quality of service than a simple Internet or digital television link.